So, as you can see, Snorlax gets murdered by DynamicPunch from Machamp without Bulk Ups. If you want to Bulk Up once and let Machamp's HP drop, Sitrus Berry (if chosen) activates, bringing Machamp's HP back up (if you don't have a Sitrus Berry, Leftovers still gets some HP back). DynamicPunch, Snorlax is gone and Machamp is at 50% HP and +1 Atk / Def. Simple enough.